Whistleblower Edward Snowden is seen on a screen as he delivers a speech during the Roskilde Festival in Roskilde, Denmark in June. On Thursday, Mr. Snowden and hardware hacker Andrew 'Bunnie' Huang unveiled a design for a device that lets users tell when their iPhone's antennas are transmitting, which could indicate that someone, such as a foreign government, is listening in.

Three years after he exposed a series of massive government surveillance programs, Edward Snowden is using his expertise to help journalists concerned about government surveillance.

Teaming up with Andrew “Bunnie” Huang, a hacker known for modifying consumer products such as an Xbox, he’s working on a device that alerts iPhone users to when their phones' internal radio antennas are transmitting.

By wiring into the iPhone’s circuitry through its SIM card slot, which stores a user's personal data, the device could alert users, particularly journalists, about when someone, such as a foreign government, is listening in.

The device, which is still in the design stage, is also intended to be a much more trustworthy method of determining whether the phone’s radios are off rather than the “airplane mode,” which can be hacked and made to look as if it is not transmitting.

Even turning an iPhone off entirely isn’t foolproof, because some malicious software can make the phone appear to be off when it is actually transmitting data, Snowden claimed in a 2014 NBC interview.

For journalists, especially those working in foreign countries, the phones’ ubiquity – as portable cameras, recording devices, and notepads – make them particularly vulnerable to sophisticated spying technology, Snowden and Mr. Huang say.

In a report last year, the Pew Research Center found that two-thirds of the US members of the nonprofit Investigative Reporters and Editors organization said they suspected the American government has collected data about their phone calls, emails, and online communications.

“I figure everything I do is tracked, watched, stored and shared and if ever the government, or hackers, wanted to know or expose everything I do, they could and would,” one journalist told Pew. “Scary, but I’m resigned to it.”

For some journalists, those concerns had affected how they communicated with their sources, Pew found.

“Our approach is: state-level adversaries are powerful, assume the phone is compromised,” he told Wired. “Let’s look at hardware-related signals that are extremely difficult to fake. We want to give a you-bet-your-life assurance that the phone actually has its radios off when it says it does.”

They point to the case of Marie Colvin, an American war correspondent who died four years ago in shelling by government forces in the Syrian city of Homs. Ms. Colvin’s family has filed a wrongful death suit against the Syrian government, alleging that they tracked her movements by monitoring her electronic communications.

Snowden's iPhone device, known as an “introspection engine,” aims to prevent surveillance by alerting a user through either messages or an audible alarm to when the radios were transmitting anything when they were intended to be off.

The device would work by using wires run through the SIM card slot to examine electrical signals from the phone’s two antennas to see if they were transmitting, Wired reports. The SIM card would be moved to another location to make that possible.

On Thursday, Snowden and Huang released a detailed paper explaining the device. Now, they hope to create a prototype and eventually a supply chain in China that could provide the modified iPhones to newsrooms and individual journalists.